i. Field of the Invention
This invention concerns a novel process for producing phosphoric monoesters. More specifically, it relates to an extremely advantageous production process for phosphoric monoesters that can conduct the reaction smoothly, provide satisfactory separation and purification of the reaction product and effectively reuse the recovered phosphoric.
ii. Description of the Prior Art
Phosphoric esters of organic hydroxy compounds have been utilized in diversified field of applications such as detergents, fiber processing aids, emulsifiers, rust preventing agents, liquid ion exchangers and pharmaceutical medicines.
Heretofore, a process for reacting phosphorus pentaoxide with an organic hydroxy compound has been known as the process for industrially producing phosphoric esters, in which the reaction product comprises an equimolar mixture of a phosphoric monoester (II) and a phosphoric diester (III) (hereinafter, the mixture is referred to as a sesquiphosphate). ##STR1## (wherein R' represents a residue of an organic hydroxy compound).
However, the phosphoric monoester and phosphoric diester are greatly different from each other with respect to the physical properties. For instance, while alkali metal salts of monoalkyl phosphates of long-chained alkyl alcohols are excellent as detergents since they are water soluble, have satisfactory foaming and cleaning performance, as well as are less toxic and thus less irritative to skin, dialkyl phosphates are scarcely soluble in water and have no substantial foaming performance. Rather, the dialkyl phosphates show foam suppressing property and, accordingly, said sesqui-phosphates containing monoalkyl phosphates cannot be used as highly foaming detergents.
In view of the above, there has been high demand to industrially produce only phosphoric monoesters selectively with safety and ease, and several processes as described below have been reported.
(1) A method of hydrolyzing monophosphorodichloridate obtained by reacting an alcohol and phosphorus oxychloride (refer to Methoden der Organischen Chemie, edited by K. SASSE, Vol. 12/2, p 163-164, and Japanese Patent Laid-Open No. 64226/1975).
(2) A method of previously adding from 0.5 to 3 mol of water per one mol of phosphorus pentaoxide to an alcohol and then reacting phosphorus pentaoxide (refer to Japanese Patent Publication No. 14416/1966).
(3) A method of reacting orthophosphoric acid and phosphorus pentaoxide with an alcohol (refer to Japanese Patent Publication No. 6730/1967).
(4) A method of reacting an alcohol with a condensed phosphate (polyphosphoric acid) (A. K. Nelson, et at, Inorg. Chem. 2. 775 (1963), or F. B. Clarke, et at, J. Amer. Chem. Soc., 88, 4401 (1966) and Japanese Patent Publication No. 26492/1968).
However, these methods have the following drawbacks and are not satisfactory as industrial processes.
The method (1) produces 3 mol of hydrogen chloride for preparing one mol of monoalkyl phosphate, which results in problems for the disposal of hydrogen chloride and in view of the working circumstances, as well as since an alkyl chloride is by-produced due to hydrogen chloride it is difficult to obtain a satisfactory yield of the monoalkyl phosphate.
In the processes (2) and (3), the ratio of the mono-alkyl phosphate can be increased by increasing the amount of water or othophosphoric acid, when only the ratio between the monoalkyl phosphate and the dialkyl phosphate is taken into consideration. However, the reaction ratio of phosphorus is reduced to increase the production amount of orthophosphoric acid. Incorporation of orthophosphoric acid to a final product gives an undesired effect depending on the application uses to thereby restrict the field of applications and reducing the commercial value of the product. For example, in a case of using a monosodium salt of a monoalkyl phosphate derived from a long-chained alkyl alcohol in the form of paste like detergent, the presence of orthophosphoric acid in a great amount will lead to the deposition of disodium phosphate, which is undesirable in view of the use.
The process (4) can selectively produce the monoalkyl phosphate. However, since the amount of orthophosphoric acid by-produced in the reaction substanially agrees with the reciprocal of the average condensation degree of a polyphosphoric acid, orthophosphoric acid is inevitably incorporated into the product to resulting in the problems as described above.
Accordingly, these prior processes can not produce monoalkyl phosphates selectively at a high purity, for example, with no incorporation of orthophosphoric acid.
By the way, as a method of removing orthophosphoric acid from the mixture of a phosphoric ester and orthophosphoric acid, only the method of Nelson et al for extracting phosphoric esters to remove orthophosphoric acid by the use of an ethyl ester has been known (Inorg. Chem. 2, 775 (1963)). However, since ethyl ether is a low boiling point solvent having an extremely low flash point and further, peroxide is gradually resulted, it may cause an explosive danger if the peroxide is accumulated and gives undesired effect to the human body, thereby inviting an industrial problem.
The present inventors have made a study on various solvents other than ethyl ether. However, it has been found that use of etheric solvent such as diisopropyl ether is not preferred since it involves the danger of explosion due to the accumulation of peroxide, although the removal of orthophosphoric acid is possible as in the case of ethyl ether. Furthermore, other solvents could not provide a satisfactory result in a single solvent system since the phosphoric ester itself is a strong emulsifier resulting in the formulation of a stable emulsion system as a whole.